PETROLEUM — FIELD 239 



temperature for long periods became available, thermal cracking op- 

 erations were pressed to higher and higher conditions of severity 

 with respect to pressure and temperature. The severe cracking con- 

 ditions produced more marked improvement in the hydrocarbon 

 structure of the cracked gasoline. It had better antiknock character- 

 istics. 



At this point in our historical review the concept of "octane num- 

 ber" should be established. The previously used "benzol equivalent" 

 was proving to be unreliable and not suflficiently reproducible for a 

 test of a property which was becoming such an important char- 

 acteristic of automobile gasoline. Benzol was so high in antiknock 

 that it was too sensitive a blending agent and Pennsylvania straight- 

 run gasoline, the zero of the scale by definition, was not invariable and 

 reproducible. The octane-number scale was substituted instead and 

 used as its "100" the chemical compound 2,2,4 trimethylpentane (iso- 

 octane) , and as its "0" normal heptane. This scale was reproducible 

 by blending these compounds and when coupled with the improve- 

 ment and standardization of the test engine resulted in a more satis- 

 factory fuel-rating procedure than had been heretofore available. 



In the refineries of the country, aided by the advances in steel metal- 

 lurgy, the antiknock characteristics of motor gasolines were being 

 pushed to higher and higher levels as new units came on stream. 

 Further, the use of additives for improvement of antiknock received 

 extensive investigation. Various organic amino compounds were 

 used with indiiferent success to increase octane number. Iron and 

 nickel carbonyls were more successful from the octane-number im- 

 provement standpoint but caused objectionable rust deposits within 

 the engine. Finally, tetraethyl lead was discovered, proved satis- 

 factory, and extensive manufacturing facilities were put into oper- 

 ation to help further the improvement in engine and fuel perform- 

 ance. 



The "octane race" of the thirties was on in dead earnest. Auto- 

 motive-engine designers moved their compression ratios higher and 

 higher to get mucli more power and performance out of smaller en- 

 gines. Figure 3 gives an interesting comparison for a representative 

 passenger automobile. From this it can be seen that during the thirties 

 the performance characteristics of the typical passenger car of ap- 

 proximately the same total weight improved considerably as a result 

 primarily of the increase in compression ratio and the improvement 

 in the quality of the motor fuel available for use in the higher com- 

 pression engine. 



At this point in our technical historical study, the issues become 

 somewhat obscure as so frequently happens whenever history is re- 

 viewed, whether it be the history of people, cities, or nations. The 

 automobile-engine designer had slowed in his advance to higher 



